Inductive power transfer is capable of providing electric power across a significant space to often moving apparatus without a physical connection for the electricity (such as sliding or rolling contacts). It can be carried out at low or high frequencies, in a loosely or a tightly coupled configuration, and with or without magnetically permeable materials.
We have described such a system in our U.S. Pat. No. 5,293,308 (and in the corresponding International application filed as PCT/GB 92/0220).
Advantages of the preferred loosely coupled inductive power transfer means over various tightly coupled transfer arrangements include that:
(a) Effective transfer is possible across a larger space, thus the primary and the secondary need not be constrained in space to move within such close limits. PA1 (b) The larger area results in a lower peak power density or a less tightly focused field to carry power, which is less hazardous and places less stress on components or on incidental objects within the flux field. PA1 (c) The pick-up coil need not surround the primary conductor so a system can be constructed in which a flat receiving surface containing secondary windings may be brought near another flat surface containing one or more embedded transmitting (primary) conductors, so permitting much freedom of movement for vehicles over a roadway, for example. PA1 (d) Useful power control means applied to the secondary side may be implemented by shorting the secondary coil (which is generally a resonant inductor) without material effect on currents in the primary side. A shorted secondary coil has little effect on primary current flow, so unaffected primary current can reach another consumer further from a power supply.
Exploiting the above advantages of loosely coupled inductive power transfer systems to utilise IPT in an optimised way uncovers the inherent disadvantages between loosely and tightly coupled systems, mainly that the available power may be limited and that secondary pick-up coils are large, expensive, have unnecessary ohmic resistance, and have large magnetic fields of their own when in use. Means to make the transfer process more effective across wider gaps are required and therefore there is a need to enhance the ability of secondary windings to collect as much of the available flux as possible.